False widow spiders have been in the UK since the 1870s but have "rapidly spread" throughout the south of England in recent years.
They are one of only around a dozen spiders in the UK capable of biting humans, but are not aggressive and will only bite if they are prodded or squashed, or trapped in clothing.
Despite several dramatic cases recently, the bites are not life threatening and most people will not have a bad reaction.
The name 'false widow' immediately provokes fear, even though false widows are "nothing like as dangerous as black widows," according to Lawrence Bee of the British Arachnological Society.
Will usually only attack when their web is disturbed or they feel threatened
Bite itself is not normally felt but is followed by "varying levels of burning, discomfort or numbness and associated swelling in the area surrounding the bite"
As with a bee or wasp sting, the seriousness depends on the individual's reaction to the bite
If you suspect an allergic reaction, or symptoms are more serious, you should seek medical attention
Symptoms usually disappear within one to three days
First sighting was in the 1870s in the South West
Believed to have been introduced in fruit crates imported from the Canary Islands
Frequency of sightings has been increasing since the 1990s
Now "well distributed" in the southern half of England
Likely that this rapid spread is "at least partly a response to a changing climate"
False widows have a large body ranging from 7mm to 14mm
Body has a dark brown, glossy appearance with a distinctive white pattern on top
They like dry, undisturbed environments like sheds and outbuildings
Active at night and normally hidden during the day
If you find one, there are likely to be more living in the same place
WHAT TO DO IF YOU FIND ONE
The best thing to do is to leave it alone. They pose no threat if undisturbed.
If you do want to move it, use a cup and piece of card to catch it and move it away from your home
Try to avoid touching them
Sources: Natural History Museum, British Arachnological Society